Macrophages enhance the activation of lymphocytes by cell contact and in part through the production of mediators such as "lymphocyte activation factor" (LAF). Our findings support the hypothesis that activated macrophages which are more mobile and more tumoricidal also produce more LAF. Supernatants of antigen activated lymphocytes contain a lymphokine ("MAF") that activates macrophages and increases their LAF production, thus amplifying the immune response. Other agents with adjuvant effects such as muramyl dipeptide, water soluble extract and interphase material from mycobacteria also stimulate LAF production. LAF production was also spontaneously increased above normal in several abnormal mouse strains, namely, nu/nu and young NZB/NZW mice. Conversely, macrophages produced less LAF in mice injected with malaria, children with far advanced cancer and anergic adults with advanced Hodgkin's disease or tuberculosis. Thus, the degree of LAF production by macrophages can be used to determine their functional state. The relationship and characteristics of mitogenic factors made by lymphocytes and monocytes are being compared by separating these populations of mononuclear cells, using stimulants specific for one or the other cell type and by chromatographic purifications. BIBLIOGRAPHIC REFERENCES: Oppenheim, J.J., Shneyour, A. and Kook, A.I.: Enhancement of DNA synthesis and cAMP content of mouse thymocytes by mediator(s) derived from adherent cells. J. Immunol. 116: 1466-1472, 1976. Meltzer, M.S. and Oppenheim, J.J.: Bidirectional amplification of macrophage-lymphocyte interactions: enhanced lymphocyte activation factor production by activated adherent mouse peritoneal cells. J. Immunol. 118:77-83, 1977.